1. Field of the Invention
The present invention relates to a yoke forming a magnetic circuit of a rotary electric machine such as a starter mounted on a vehicle.
2. Description of the Related Art
For example, there are various techniques for producing a yoke as a component of a rotary electric machine (or a motor) such as a starter to be mounted on a vehicle. Japanese patent laid open publication No. JP S64-60247 has disclosed such techniques, drawing and blanking and caulking. The former technique performs the drawing process to make a cylinder shaped body from a steel plate. The latter technique performs the blanking process to make a cylindrical shaped body and then performing the caulking process to caulk both ends of the cylindrical shaped body.
However, both the conventional techniques require to perform coating to cover the surface of the cylinder shaped body with a corrosion-resisting thin film, or to perform electroplating, and then to cut the corrosion-resistance thin film of both end surfaces of the cylindrical shaped body in order to obtain electric conductive surfaces. Thus, the conventional techniques require plural manufacturing steps to make a yoke.
Recently, there is a trend to reduce in weight a starter in view of fuel consumption reduction. One technique to reduce the weight of a starter decreases the entire volume of a motor based on a high reduction ratio. However, although such a technology further reduces the thermal capacity thereof in addition to the reduction of the size or volume of the motor, there is a limit to the magnitude of the reduction. Thus, there still remains a technological problem how to enhance the radiation capability of the motor.